While this sounds somewhat like the call to the Feast of Tabernacles, the whole seventh month (Tishri) is consumed in various celebrations following the olive harvest: Trumpets (1st), the Day of Atonement (10th) and then Tabernacles (15th). The psalmist mentions both the New Moon and Full Moon rituals, as well. Then he takes on a prophetic note in this celebration: Can we just this once remember to celebrate the Lord by obeying Him? In doing so, he focuses on commemorating the Exodus.
The psalm begins with a lyrical call to observe a ritual of celebratory music. He goes on to note that there are other observances commanded in the ritual law to stir up the natural joy of gratitude we owe to our Creator and Lord for all His blessings. He notes the first deliverance was before Israel was a nation, when Joseph became viceroy in Egypt where people speak a totally different language. God placed Joseph there to protect Israel from His wrath falling on the various nations in Canaan Land.
And when the time came, Israel was rescued from slavery. When God saw the fruit of things was ripe, He answered their prayers and put them through the necessary testing any sheikh used to prepare his servants. They were so used to life in Egypt that they weren’t yet ready to bear His revelation to the world.
Asaph focuses on the signal element of loyalty in the divine call at Mount Sinai. The first of the Ten Commandments is entirely normal. A covenant of adoption as the family of any great sheikh required that they swear blood loyalty to him, the same as he would require of natural born kin. In return for that loyalty, He will supply more than they could take in, so open wide!
But they refused to keep faith in Him. His revelation was the ultimate reality of all things, but they preferred their own imaginations. So He let them have what they really wanted, and with it all the suffering of human moral blindness.
When will they ever wake up? If only they could render a genuine commitment to Him, no enemy could come against. Even those with a rabid spite for Jehovah would be forced to kneel before His authority. Not that it would do those enemies any good eternally, but God has always been ready to humble outsiders who threaten His servants. Israel could not imagine just how great a harvest they could have, both from their work and from random wild provision.
